The nFD 50mm f1.2 is what I have been using. Love its superb optics and the small size and well built quality. It's a lens great in day time but designed for, and excels at, low light situations. Of the couple dozen or so lenses in the 50mm focal length, this is the lens I like to use the most in recent times. Wish I could get my hands on a nFD 50mm f1.2L or the 55mm f1.2 Aspherical. Those two should be even better optically, but I have never used them, unfortunately.
Allen Gardens at night - NEX-6 & Canon nFD 50mm f1.2 @ f1.4, ISO 3200. Click for larger.
Lamp Post - NEX-6 & nFD 50mm f1.2 @ f1.4, ISO 1600